For a good, non-padi basic book try Jeppesen's Open Water Sport Diver Manual, avaliable from amazon.com. It's alot more level and neutral than the PADI/NAUI texts. And it tells you how to do things, something I have not found in some other introductory books for other certifying agencies. A review at amazon says "Jeppesen, the origional publisher, publishes all the texts that airplane pilots use". They also have a Jeppesen's Advanced Open Sport Diver Manual, which I also recommend. The open water is $14, and the advanced is $25 so they won't set you back too much.
On the total other end of the spectrum I've been reading Wienke's Technical Diving in Depth, which is really applicable to all divers wanting to understand more about decompression (remember every dive is a decompression dive), as well as waves, tides, Ocean currents, diving maladies and drugs, computer algorithms, etc. Parts of book are very complicated (including the last half of the first chapter), and Wienke goes into some excessive detail, beginning with the big bang to explain tides (he gives a brief run through of cosmology to introduce gravity). Each chapter is self contained, so you can read them out of order and skip the ones you don't want. Each chapter has problems with solutions (not just answers) in the back. If you want to derive every equation yourself you'd need basic college physics, calculus through diff eq, and thermo. I'd just believe him. (And if you try to derive the equations, prepare to be frustrated, because he frequently integrates and doesn't tell you with respect to what variable, time, depth, etc). Still, only absorbing 25% of the material will give you a better understanding than 90% of the instructors out there.
Happy reading and good luck with the class.
-Katrina